1. Field of Invention
This invention relates to measurement of soil moisture, and to control of irrigation systems.
2. Description of the Related Art
Measurement of soil moisture is useful for minimizing the amount of irrigation water applied to growing plants, and for optimizing plant growth. Present uses of such systems are mostly confined to agricultural activities in dry climates where significant quantities of groundwater are required to maintain the crops. Minor use of such systems is made by homeowners, and some golf courses, wishing to conserve and optimize their use of water for grass. Soil moisture measurements are also used in groundwater studies. Soil moisture changes are also measured under and around waste sites, hazardous liquid lagoons and landfills in order to detect leaks.
The most common soil moisture sensor in use is a small gypsum block having a volume of about one cubic inch and encasing two metal electrodes. Gypsum absorbs moisture and the resistance across the two electrodes provides a measure of the moisture content within the gypsum. The gypsum blocks are buried in the soil at the depth where moisture content sensing is required, and the two wires from the electrodes are brought to the surface of the ground to facilitate resistance measurements. Gypsum blocks provide an indirect soil moisture indication since resistance measurement across the electrodes is influenced only by the moisture within the block. The block has to absorb moisture from the surrounding soil in order for this sensor to work. The degree to which this sensor will work depends on the packing density of the soil surrounding the block. Air space surrounding the block reduces its efficiency. Because gypsum has a strong affinity for water, the blocks tend to hold their absorbed water even when the surrounding ground is drying. This effect reduces sensitivity to soil moisture changes, in particular to drying soil, since the strong affinity for water exhibited by gypsum inhibits the block's capacity to lose water and hence indicate drying conditions. Since the gypsum must absorb water in order to detect increasing moisture content, and must dry out in order to detect reduced soil moisture, some time is required for these events to occur. This is especially true in the drying stage, and a considerable delay can occur between the time at which the soil has reached a certain level of dryness and the indication given by the gypsum block. With time, gypsum blocks disintegrate within the soil. The two electrode system used in these blocks essentially provides a measurement of the resistance of the electrode to gypsum contact along with the resistance of the gypsum. If the contact of either electrode with the surrounding gypsum changes, then a corresponding change in the resistance measurement, unrelated to soil moisture content, occurs.
Soil moisture is also measured using two electrodes placed in the ground. As with the electrodes in the gypsum block, this is a measurement of the resistance to ground, often called electrode resistance, of each of the electrodes, added to the resistance of the ground between the electrodes, which is usually fairly small compared to the electrode resistance since the current has a large volume of ground through which it can flow. Thus, the measurement of soil moisture using two electrodes is essentially a measurement of the electrode resistance. Any disturbance of the electrodes causes a change in the resistance measurement unrelated to the soil moisture content. U.S. Pat. No. 3,944,916 to Tillander (1976) proposes using two such electrodes to measure the resistance of soil in a plant pot. U.S. Pat. No. 4,026,467 to Chevreliere (1977) also uses two electrodes to measure the resistance of the region around the roots of a plant.
The measured resistance of two electrode systems also depends on the resistance of the wires used and changes when the length of the wires, and hence their resistance, is changed, or when any other resistance is included in the circuit. In addition, electrochemical effects result in voltages at the metal electrode to ground contact and must be accounted for.
An electrode system is proposed by Koller, West German Patent 2101301, (1972), where two electrodes are inserted into the soil to measure contact resistance. U.S. Pat. No. 2,768,028 to Robinson (1956) describes a sensor made of carbonaceous, electrical conducting material for measuring soil moisture. However, the patent does not describe the reason for the carbonaceous material nor the physics of the sensor.
Soil moisture is also measured using nuclear devices. These devices emit a stream of neutrons which interact with the hydrogen in the water molecules. Another technique which is now being used is called Time Domain Reflectometry which uses changes in the dielectric properties of soil to indicate moisture content changes. A similar method is called Frequency Domain Capacitance. However, these instruments are expensive and not appropriate for general irrigation purposes.
Some of the simpler, and less expensive devices include heat dissipation sensors and sensors containing fiber glass which work in essentially the same way as gypsum block sensors. Another method of measuring soil moisture is to use a Tensiometer. These devices have a porous medium and measure the suction strength created as the porous medium sucks water from the soil. As the soil dries, the suction becomes stronger. Comparison tests between a Tensiometer and the sensor described in this patent application show this sensor to be much more sensitive to moisture changes than the Tensiometer. Tests have also been conducted comparing Time Domain Reflectometry and gypsum blocks with this sensor, showing this sensor to be much more sensitive to moisture content changes that these other two methods.
The resistivity of the ground depends mostly on the soil moisture content and the salinity of the moisture, and can be measured using a four electrode array (Applied Geophysics, W. M. Telford, L. P. Geldart, R. E. Sheriff and D. A. Keys. Cambridge University Press, 1976; pages 654-661). With this system, in order to obtain good contact with the ground, the electrodes are one or two feet long. Two electrodes are used to pass electric current into the ground and the resulting voltage is measured across the other two electrodes. It is common with the four electrode systems to pass alternating current across the current electrodes, a procedure which removes the errors due to natural potentials at the electrode-soil contact along with naturally occurring noise, such as Telluric currents. An equation can be developed to calculate the resistivity of the ground involving the geometry of the electrode array, the amount of current injected into the ground and the resulting voltage. Often a linear array of equal spaced electrodes is used to measure resistivity. However, with this system, the resistivity measurement involves a volume of ground whose dimensions are related to the geometry of the electrode array. With the equal spaced electrode array, the volume of investigation is related to the electrode spacing, and a large volume of ground is sampled during measurements. Thus, very small electrode spacings are required to obtain resistivity values of small regions, such as around the root zones of grass. This means that small electrodes are required, thus increasing the problems of electrode contact with the soil and thus the injection of current into the ground. In addition, with the four electrode system, the lack of focusing of the electric currents allows these currents to flow in the more conductive regions of the soil. Thus, if shallow soil moisture is required for grass irrigation purposes and the ground a few inches below the root zone of the grass is wet, then the current will tend to flow in this wet ground, reducing the resistivity value measured and lowering the sensitivity of the resistivity measurement to the upper, and dryer, portions of the soil. A resistivity sensor is needed, therefore, where the volume of influence of the sensor is confined to the region of interest with good contact being made between the electrodes and ground while maintaining the advantages of the four electrode system.
If moisture measurements are attempted within a soil rich in clay, only very small changes are recorded during the time it takes grass to become water deficient. This is because most of the moisture is locked up in the fine pores of the clay and unavailable to the plant. If, however, moisture measurements are made in a small volume of sand placed at the root zone of the grass within this clay rich soil, then the moisture content of the sand changes dramatically, and provides a very sensitive indicator of the moisture requirements of the grass. The sensor described in this patent application can have its volume of influence reduced so as to be able to accurately record moisture changes in this small volume of sand.
Japanese Publication JA 61-173134 to Itagaki describes a moisture or temperature detecting element wherein a pair of electrodes are spirally formed on a ceramic circular body. A conductive material whose resistance changes with atmospheric gas occupies the space between the electrodes.
U.S. Pat. No. 2,907,841 to Campbell describes a bed wetting signal device wherein metal foil electrodes are placed in alternating fashion on a flexible polyethylene support for shorting by liquid or wet clothing. U.S. Pat. No. 4,801,865 to Miller et al describes a soil moisture probe wherein contact points are bridged by moisture. U.S. Pat. No. 2,636,962 to Bouyoucos describes a soil moisture meter that includes a variable resistance moisture absorption unit that is connected to be a portion of a resistance measuring bridge circuit. U.S. Pat. No. 4,652,811 to Kwiat et al describes a soil liquid content measuring device wherein spaced electrodes are placed in the soil, a voltage is applied to the electrodes, and the resulting electrode current flow is measured. Soviet Union document 0822012 by Badinter describes a humidity sensing device for use in a gas wherein uninsulated conductors are wound on a former that is coated with lithium chloride.